A very common technique of packaging integrated circuit chips or dies is to mount the integrated circuit chip on or in some type of substrate carrier by various types of mountings, such as control collapse chips (C-4), to form a chip component or chip module, and to mount the chip module or component to a circuit card, such as by a ball grid array (BGA) structure. (The chip and substrate are sometimes referred to as a ball grid array.) One material which has been conventionally used as a substrate for mounting the I/C chip or die is an inorganic ceramic material, such as alumina. These inorganic substrates are relatively rigid and, hence, even when an I/C chip is mounted thereon or therein, any difference in the thermal coefficient of expansion (CTE) between the chip and substrate does not result in any significant warpage of the substrate. Hence, the substrate and the module itself are substantially planar and amenable to attachment by a BGA to circuit board. Such a structure is described in the magazine IBM Microelectronics Second Quarter 2000, pages 26-36 (a copy of which is attached hereto and incorporated by reference). In such case, there is not any significant problems vis-à-vis CTE mismatch in making such attachment since the stress has been contained between the die and the substrate mounting the die, and a relatively flat surface is available to mount the component, including the carrier substrate, onto a relatively flat circuit board or card.
However, if one wishes to use more flexible, organic substrates on which or in which to mount the die, as described in the above entitled article, then the differences in the CTEs of the various material can result in a warpage of the component substrate, as described on pages 26 and 27 of the above entitled article. Thus, while certain techniques can be employed to somewhat minimize the warpage of the substrate, nevertheless this warpage cannot be totally eliminated and the techniques to avoid such warpage introduce other design and structural problems into forming the die containing component. Therefore, it is desirable to provide a method and structure to compensate for bowing of the component or I/C chip module due to CTE mismatch and other factors, thus allowing greater freedom of design but without unduly stressing the BGA connections.
A structure and technique for forming an I/C chip module and circuit card construction is provided. The structure includes an I/C chip module having a flexible substrate with first and second opposite sides. An I/C chip is mounted in, or on the first side of the substrate. The substrate is curved concave with respect to the second side which is the result of CTE mismatch between the I/C chip and substrate. A flexible circuit card is also provided having first and second opposite surfaces. Solderballs are interposed between the second side of the substrate and the first surface of the circuit card in a ball grid array configuration. The solderballs interconnect the substrate to the circuit card. The curvature of the I/C chip module and CTE differences cause the circuit card to curve corresponding to the curve of the module. A heatsink is provided which is in thermal contact with the I/C chip and positioned to place pressure on said I/C chip module in a direction to decrease the curvature. A rigid backing member is also provided to retain the circuit card which, thus, generates a curved space between the backing member and the second surface of the circuit card. A force generating structure, preferably in the form of a spring, such as a leaf spring, is provided to apply pressure to the I/C chip module against the backing member. A compliant member is interposed between the backing member and the second surface of the circuit board filling portions, or all of the curved space therebetween, most critically between the board and backing member in the curved region near the component. By providing the compliant member, when force is applied to the heatsink, the compliant member will flow to conform to the space between the backing member and the second surface of the circuit board, thus evening pressure distribution and preventing the straightening out of the circuit board. Such a straightening out would apply additional stress to the ball grid array structure and could in some cases result in failure of the joints. Rather, the curved structure of the I/C chip module and the corresponding curved structure of the circuit board are maintained to minimize the stress on the solderballs in the ball grid array. In this manner, relatively higher pressure can be utilized without damaging the joined component; it is known that relatively higher pressure contributes to improve thermal contact, better cooling, and thus the invention allows higher power devices to be used with greater reliability and functionality.